Middle Pleistocene re-organization of Australian Monsoon

The sensitivity of the Australian Monsoon to changing climate boundary conditions remains controversial due to limited understanding of forcing processes and past variability. Here, we reconstruct austral summer monsoonal discharge and wind-driven winter productivity across the Middle Pleistocene Transition (MPT) in a sediment sequence drilled off NW Australia. We show that monsoonal precipitation and runoff primarily responded to precessional insolation forcing until ~0.95 Ma, but exhibited heightened sensitivity to ice volume and pCO2 related feedbacks following intensification of glacial-interglacial cycles. Our records further suggest that summer monsoon variability at the precessional band was closely tied to the thermal evolution of the Indo-Pacific Warm Pool and strength of the Walker circulation over the past ~1.6 Myr. By contrast, productivity proxy records consistently tracked glacial-interglacial variability, reflecting changing rhythms in polar ice fluctuations and Hadley circulation strength. We conclude that the Australian Monsoon underwent a major re-organization across the MPT and that extratropical feedbacks were instrumental in driving short- and long-term variability.


Primary productivity and bottom water oxygenation
We use visible light relative absorbance band depth at 660 nm (RABD660 in ~250 yr resolution) derived from color reflectance spectroscopy, XRF-scanner derived logarithmic ratio of manganese and sulfur (Log(Mn/S) in ~200 yr resolution) and spectral gamma ray derived uranium (U) concentrations (~1 kyr resolution) to monitor variations in primary productivity and in bottom water oxygenation at Site U1483.
The relative absorbance band depth at 660 nm (RABD660) from color reflectance spectroscopy is directly related to the concentration of chlorins in the sediment, which are a decay product of chlorophyll alpha from marine primary producers (Material and Methods).
Detailed studies of Total Organic Carbon (TOC) and chlorin concentrations measured in discrete samples from piston core MD01-2378, located 0.8 nmi northwest of Site U1483, revealed a marked glacial-interglacial variability over the last five glacial cycles [4][5] . The glacial-interglacial variability in chlorin concentrations closely matches that of the RABD660 record extracted from the shipboard spectrophotometry measurements in the same core Numerous studies have highlighted the association of elevated U concentrations with organic-matter-enriched marine sediments 9 . Uranium concentrations are frequently used as proxy indicators of organic matter preservation in marine sediments and U enrichment has been suggested as a tool to characterize suboxic marine environments [10][11] . On a global scale, more than three quarters of the total dissolved U riverine flux to the ocean is deposited in suboxic continental margin sediments that constitute the largest global U sinks 12   At Site U1483, the downcore decrease in porosity and water content markedly affects the upper ~60 m (0 to ~600 ka) and is clearly expressed in the shipboard GRAPE density data 18 and the XRF scanner chlorine (Cl) record (not shown), which is related to the salt content in the pore water.
Supplementary Figure S3. U1483 proxy records of marine primary productivity and bottom water oxygenation, derived from reflectance spectroscopy, XRF-scanner and spectral natural gamma ray data. A. RABD660, relative absorption band depth at 660 nm (proxy for chlorin concentration) from shipboard reflectance spectroscopy 13 . B. Bromine area counts per second (cps) from XRF-scanning. C. Logarithmic ratio of manganese and sulfur (Log(Mn/S)) from XRF-scanning. D. Uranium concentration estimates (ppm) from shipboard spectral gamma ray data [18][19] . E. U1483 benthic foraminiferal δ 18 18 . B. Logarithmic ratio of manganese and sulfur (Log(Mn/S)) from XRF-scanning. Note long-term increasing trend after ~1 Ma related to compaction and associated changes in pore water content in the upper ~100 m of the sediment succession. C. Uranium concentration estimates (ppm) from shipboard spectral gamma ray data [18][19]

Riverine terrigenous discharge
We used the sum of XRF-scanner K-alpha area counts per second (cps) of the typical clay mineral-derived elements aluminum (Al), potassium (K), Fe and titanium (Ti), abbreviated as Terr, as a proxy for the terrigenous sediment-component, mainly originating from the Australian continent via riverine transport into the Timor Sea. Comparison of XRF-scanner area cps counts, measured with the same settings on the same instrument in two neighboring cores, to absolute elemental concentrations based on 20 fused beads XRF analyses of decarbonatized sediment indicated clear linear relationships with R 2 of 0.74 for Al, 0.93 for K, 0.98 for Fe and 0.98 for Ti 22 . We did not include silicon (Si) and zirconium (Zr) in Terr, since these elements commonly occur in quartz and zircon grains, which are subject to sorting processes during transport and may be wind-transported during glacials. This is also partly the case for Ti, which is included in Terr, but does not significantly influence the record due to its low area cps values and small deviation from other terrigenous elements. The Si record nevertheless exhibits high similarity to the Terr record, suggesting that the contribution of windblown quartz grains and biogenic opal to the total Si record is low. Rubidium (Rb), a common replacement of K in the clay mineral illite, exhibits an almost identical trend to that of the other clay mineral derived elements (Supplementary Figure S6). However, Rb was not included in Terr, since it is the only clayderived element that was not measured with the 10 kV setting of the XRF scanner and, thus, area cps counts are not comparable. We normalized Terr using its log-ratio to calcium (Ca) that is mainly derived from the biogenic carbonate of marine plankton. This approach is commonly used in carbonate-rich environments, where no indication of significant changes in carbonate dissolution or in carbonate productivity are detected 3,23-26 . We, therefore, calculate the logarithmic ratio of clay-mineral bound terrigenous elements normalized to Ca using the formula: Log((Al_area cps + K_area cps + Fe_area cps + Ti_area cps)/Ca_area cps), abbreviated as Log(Terr/Ca) We also note that the orbital-scale variability of the riverine sediment discharge proxies covary with that of the Log(Al/K) ratio, which is associated with changes in chemical weathering conditions within the catchment areas of the northwestern Australian rivers (Supplementary Figure S7). The K-enriched clay mineral illite is generally a weathering product in temperate and arid climates, where physical weathering is dominant. By contrast, Al-rich kaolinite is usually a product of chemical weathering in humid climates [27][28][29] .
Accordingly, high Log(Al/K) values correlate with higher rates of soil weathering during periods of increased precipitation in the source area and low Log(Al/K) values correspond to lower weathering rates associated with generally dryer conditions and/or higher erosion rates under higher rainfall seasonality.
Carbonate-free basis normalization [element abundance x 100 / (100 − CaCO3)] was additionally used to evaluate the influence of variations in carbonate accumulation rates on terrigenous elemental concentrations. Despite being influenced by the weathering regime we consider K as a representative terrigenous element, since K is a key component in illite-rich clay mineral assemblages derived from NW Australian rivers 30 Figures S11 and S12), suggesting that illite is a representative component of clay mineral assemblages derived from the NW Australian source area.
Fluctuating sea levels may determine the discharge of terrigenous sediments offshore major river and delta systems with predominant deposition of marine carbonates during sea level highstands and increased input of fluvial sediment load during sea level lowstands, when the coastline was more proximal and rivers drained more directly over the shelf edge 30 .
However, the main increase and maximum in terrigenous discharge along the NW Australian margin occurred in the late stage of glacial terminations and during interglacials, when the sea level was rising or close to maximum. The occurrence of peak monsoonal sediment discharge during interglacials, when the distance between Site U1483 and the adjacent river mouths was at a maximum, clearly excludes proximity to the coastline as the main driver of terrigenous sediment accumulation at this site. The influence of glacial interglacial sea-level changes on the XRFscanner derived sediment composition at Site U1483 over the last 0.41 Myr, previously evaluated using Ca-normalized terrigenous elemental data, was not found discernible 3 . We relate the lack of a sea level imprint on the terrigenous sediment discharge at Site U1483 to the relatively far distance of the site location to the coastline even during glacial sea level lowstands ( Figure 1) and to the efficient transport and direct delivery of the sediment load to the Timor Sea without the development of huge flood plains and delta systems. Increases in terrigenous sediment discharge are, thus, directly associated with erosion and runoff by intensified monsoonal precipitation in the catchment area.

Cross spectral analysis
Cross-spectral analyses were performed with the Blackman-Tukey approach using AnalySeries version 2.08 37 . All data are interpolated to a constant time step close to the actual data resolution, and pre-treated by removal of linear trend and pre-whitening. All spectra use a Bartlett window and a 30% lag. The bandwidth varies between 0.0056 and 0.0117. Nonzero coherence is higher than ~0.385 (Supplementary Table S2 Figure S16). We excluded high-productivity equatorial and coastal upwelling regions along the Pacific and Atlantic eastern margins such as Sites 1012, 1020 and 1082-1084, as the evolution of regional SST at these locations is mainly driven by regional processes involving complex current dynamics that are not coupled directly to high-latitude climate evolution [46][47]  In Figure 4, we selected the NH high latitude Site 882 in the subarctic Pacific Ocean 50 , because the evolution of SST between 1200 and 800 ka in the North Atlantic exhibits contrasting trends with simultaneous cooling and warming along the northwestern and southeastern margins (respectively) of the North Atlantic Current during MIS 28 (~995 ka) 51 .
The shift from 41 to ~100 kyr cyclicity was also diachronous in these two regions 51 . The later shift to the southeast of the North Atlantic Current was attributed to the gradual southward spreading influence of NH ice sheets 51 . By contrast, the impact of high latitude climate cooling and ice sheet growth on North Pacific Ocean SST appears more straightforward during this interval. In the SH, we selected Site U1090, located in the subantarctic Atlantic, as the SST record from this site exhibits strong similarities to the spectral characteristics of Antarctic ice core climate records 50 .
Both records from Sites 882 and 1090 are based on the original U K 37 index, which exhibits a more robust relationship to annual mean SSTs than U K 37' at high latitudes [52][53] . Moreover, the U K 37 index includes the tetra-unsaturated alkenone (C37:4), which is now regarded as a sea ice proxy 54 , thus providing additional information on the regional variability of fall and winter climate conditions at high latitudes. This index may, however, bias SST estimates to colder values in contrast to U K 37', which preferentially record temperature during the favourable spring and summer growth period for alkenone producing phytoplankton. This bias may explain the strikingly low U K 37-based SST estimates at Site 882, likely associated with an increase in seasonal sea ice between ~1 and ~0.95 Ma in the Bering Sea 55 .